Laminated veneer lumber (LVL) is an engineered wood product that is fabricated from sheets of thin wood pieces (e.g., veneers) that are glued together in panels called billets. When LVL is manufactured, the veneers are oriented so that the grain in each individual sheet is aligned primarily along the length of the billet. Pieces of LVL are trimmed from the billet for use in a variety of applications (e.g., joists, headers, beams, rafters, flanges for I-joists, etc.).
FIG. 1 is a flow chart depicting a conventional LVL manufacturing process 100. The start of the LVL manufacturing process 100 depends on how the plant running the process 100 obtains the veneers. Plants may either peel and dry veneers onsite (step 102), purchase green veneers (step 104) and dry them onsite (step 108), or purchase pre-dried veneers (step 106).
After initial processing, the veneers are graded for stiffness and/or strength as shown in step 110. Generally, veneer grading is a highly automated process involving both visual grading methods and automatic grading methods (e.g., ultrasonics). The objective of grading is to permit the most efficient use of the available veneer. The lower grade veneers are used for the LVL core and the higher grade veneers are used in the LVL face.
Following grading, the veneers are laid out and prepared for pressing. An adhesive (e.g., a resin) is applied to the veneers (step 112) and the veneers are aligned or laid up (step 114). FIG. 2 shows a typical veneer lay up 200 according to the method described in FIG. 1. As shown in FIG. 2, a first veneer piece 202 having a first edge 204 is aligned next to a second veneer piece 206 having a second edge 208 so that the first edge 204 and the second edge 208 overlap. The overlapping distance is shown as reference character 210. LVL billets are produced by applying layers of veneer and adhesive sequentially. Some plants utilize modular assembly systems containing a station for each successive layer of veneer in the product.
After lay up, the veneers are pressed. During pressing (step 116), LVL is manufactured to either a fixed length using a batch press, or to an indefinite length using a continuous press. FIG. 3 shows an example of a pressing operation 300. In FIG. 3, the veneers and placed between a first platen 302 and a second platen 304. The first platen 302 and second platen 304 are pressed toward each other to form a lap joint 306. The presses are heated by electricity, microwaves, hot oil, steam, or radio-frequency (RF) waves. Press temperatures range from about 120° to 230° C. (250° to 450° F.). The exact pressing conditions are designed to bring the veneer surfaces tightly together without over-compressing the wood.
FIG. 4 shows the lap joint 306 in a finished billet 400 after it is removed from the press. Billets exiting the press may be up to 8.9 centimeters (3.5 inches) thick. Billets are produced in widths of up to 2.8 meters (4 feet). After exiting the press, the billets are visually inspected (step 118) to identify particular features (e.g., lap lengths, slip sheets).
After inspection, the billets are cut into LVL (step 120). The billets are typically cut into numerous strips based on customer specifications. The LVL is produced in lengths up to the maximum shipping length of 24 meters (80 ft). After the LVL is cut, other finishing applications may be performed (e.g., sorting, treating, stacking, stamping) as depicted by step 122.
A common challenge in veneer manufacturing is avoiding the formation of slip sheets. A slip sheet is a feature where two pieces of veneer intended to form a lap joint have failed to overlap. FIG. 5 is a top view of a billet 500 which illustrates this problem. The portion of the billet 500 shown is formed by a first veneer sheet 502, a second veneer sheet 504, a third veneer sheet 506, and a fourth veneer sheet 508. The first veneer sheet 502 and the second veneer sheet 504 have failed to overlap, forming a slip sheet depicted as reference character 510. In contrast the second veneer sheet 504 has been pressed to adhere to the third veneer sheet 506, forming a lap joint shown as a dotted line 512. Likewise, the third veneer sheet 506 has adhered to the fourth veneer sheet 508 forming a lap joint shown as a dotted line 514. Features such as the slip sheet 510 may cause the portion of the billet 500 where the veneer sheets do not properly overlap to exhibit inferior mechanical properties when compared with the portions of the billet 500 where proper lap joints have been formed. Currently slip sheets and other features are detected visually by workers who inspect the billets for visible defects (see step 120 in FIG. 1). Therefore, there is an opportunity to improve LVL manufacturing processes by developing automated methods for inspection.
In addition to avoiding slip sheet formation, LVL manufacturers also strive to optimize the length of the lap joints which are successfully created. The length of a lap joint is commonly referred to as a lap length. Lap length is currently determined visually during the inspection step 120 from FIG. 1. Lap lengths that are too short may result in the manufacture of final product that has inferior mechanical properties. On the other hand, manufacturing LVL with a lap length that is too long is not an efficient use of the raw materials in supply. Currently presses are without any direct feedback or indication of lap position. Therefore, it is common practice to operate with excessive laps to reduce the risk of product fall down.
Thus, there is a need to develop improved systems and methods for detecting features during LVL manufacturing. Specifically, there is a need to develop systems and methods for detecting lap lengths and slip sheets in billets. Such systems and methods could be used to optimize the LVL manufacturing process and to control the quality of the final product.